One of the important aspirations in the further development of modern memory technologies is to increase the integration density; consequently, achieving a reduction in the structural sizes of the memory cells on which the memory devices are based is of great importance. Conventional memory cells are produced by lithographic techniques. To reduce the memory cell size, ever shorter wavelengths are being used for illumination in such lithographic techniques in order to improve the resolution of the lithographic techniques. To further advance this technology, new resist materials continually have to be developed for this purpose and the existing techniques have to be improved. With this in mind, it can be foreseen that lithographic techniques will soon come up against their practical limits. For this reason, new methods which make it possible to reduce the size of electronic components without having to use conventional lithographic techniques are being developed. These are intended to make viable electronic components which have a size in the nanometer range and are therefore several orders of magnitude smaller than the electronic components which can be produced by present lithographic techniques.
A further aspect of increasing the memory available per unit area of a memory device is the provision of memory cells which allow higher quality information units in the form of multiple states at one location in the sense of a multilevel information storage.
In recent years, a number of microelectronic elements which have a size of a few nanometers have been described. These elements are referred to as nanoelements, and the technology for producing them is referred to as nanotechnology. The proposed elements generally have a molecular layer (monolayer) located between two electrodes. The monolayers are preferably formed by self-organization on a suitable substrate. Such elements can, in the ideal case, be reduced to sizes in the molecular range from about 0.5 to 5 nm. In general, a number of individual molecules limited by the electrode area (e.g. 10 nm×10 nm) (e.g. 100, so that the density is about 1 molecule per 1 nm2) is used for producing a memory function so as to increase the statistical certainty. These molecules are preferably located in a passive matrix or in a molecular assembly, with an assembly of molecules forming the memory function at each intersection point of the passive matrix.
In “A nanometer scale electronic switch consisting of a metal cluster and redox-addressable groups,” Nature, vol. 408, 2000, pages 67 to 69, Gittins et al. describe gold nanoclusters which have a size of a few nanometers and are functionalized with polymethylene chains. The chains bear a redox-active bipyridinium radical, so that the properties of the gold cluster can be altered as a function of the oxidation state of the bipyridinium radical. Gittins et al. describe the switching behavior of the gold nanoparticles by scanning microscopy (STM) and show that such a molecular switch has various, distinguishable states. The disadvantage of this concept is that the free radical formed by uptake of an electron into the bipyridinium radical is delocalized over only a few atoms and is therefore sensitive to oxygen or other oxidizing agents. This makes the concept described in Gittins et al. unsuitable for use in a microelectronic component.
Collier et al., in “Electronically configurable molecular-based logic gates,” Science, vol. 285, 1999, pages 391-393, describe molecular switches which can be used as logic gates. The molecular switches described in Collier et al. can be used as a Programmable Read Only Memory (PROM) cell. The molecular switches described in Collier et al. have a monolayer of mechanically interlocked molecular units (rotaxanes). The molecular units consist of a crown ether which is arranged around a chain bearing two bipyridinium radicals. In this structure, too, the free radical is delocalized over only a few atoms and is therefore very sensitive to oxygen. In addition, the switching process is not reversible.
Lee et al. describe in “Fabrication approach for molecular memory arrays,” Applied Physics Letters, vol. 82, 2003, page 645-647, molecular wires comprising phenylene-ethylene oligomers arranged as a monolayer between two palladium nanowires.
U.S. Pat. No. 5,505,879 describes charge transfer complexes between fullerenes and particular electron donors in general. The molecular ratio between the electron donors and the fullerenes is from 1:3 to 6:1, with the preferred ratio being from 1:1 to 3:1. The preferred electron donor is N,N-diethylaniline. The use of these complexes for producing semiconductor elements cannot be deduced from this prior art.